Critical examination of equilibrium constants proposed for the methylcyclohexane dehydrogenation to toluene

Abstract

Methylcyclohexane dehydrogenation to toluene and hydrogen (MTH) is an important reaction for H₂ energy based systems. Hence, it is important to have a reliable equation for the dependence of the reaction equilibrium constant on temperature. A number of such equations have been proposed in the literature, but it is unclear which of them is more accurate in representing the equilibrium of methylcyclohexane dehydrogenation to toluene. To resolve this uncertainty, we analyzed their validity based on existing experimental data. Experimentally derived equilibrium constants were found to be generally bigger than those calculated by classical thermodynamics at 300–400 °C and 3, 10, and 15 bar. Equilibrium constants calculated based on tabulated free energy of the reaction, were closer to the experimental values than those based on standard heat capacities of the reacting materials. Part of the deviation is caused by the non-ideality of the reacting components at these conditions, which is reflected by their calculated compressibility factors. The equilibrium conversions calculated based on the proposed equilibrium constants have been compared to those obtained in a set of methylcyclohexane to toluene reactions carried out in continuous flow packed bed and membrane reactors and this comparison revealed that only the experimentally derived equilibrium constants provide reasonable results.

Keywords:

• Dehydrogenation
• Equilibrium constant
• Hydrogen storage
• Membrane reactor
• Methylcyclohexane
• Toluene

Acknowledgments

Financial support from the Swiss Federal Office of Energy (Bern), during the period of carrying out the experimental work, is acknowledged.